THE OLD RED SANDSTONE OF THE ORKNEYS. 393 



present. The long axis of the Loch of Harray corresponds very closely with the crest 

 of the anticline, as on the different sides of the loch the dips are opposite, and at Brodgar 

 Bridge, at Ness in Harray, and at Dounby we have the flat or gently rolling beds which 

 occupy the summit of the arch. A transverse section of the anticline is exposed on the 

 north coast of the Mainland, from Marwick Head in Birsay to Costa Head in Evie. At 

 Marwick Head the dip is N.W. about 10°, and this continues, with occasional variation 

 and a few small faults, seen in the Bay of Birsay, to Skip Geo, just east of the Brough. 

 Thereafter, along the coast by Crustan to the mouth of Swannay Burn, the rocks lie 

 very flat, with gentle and frequently changing dips, in which, on the whole, those 

 to the east and north-east preponderate. In Costa Head the east dip is persistent, 

 and, gentle at first, constantly increases along the shore line to Burgar, and thence 

 to Aikerness Point in Evie. In this entire and perfect section no disturbance of 

 the flags is anywhere seen sufficient to indicate the existence of a fault of any im- 

 portance. 



If we traverse the Mainland along an east and west iine through its centre, the 

 result is the same. Starting at Skaill Bay, we find that the rocks are rolling, but the 

 dips are always westward. Between this and Dounby the low lands are in many places 

 covered with boulder clay, but in all the quarries the dips are west till we arrive within 

 a few yards of the village, where it rolls to north-east. More exposures can be examined 

 by following a line past the Loch of Clumly to the Bridge of Brodgar, which separates 

 the Lochs of Harray and Stenness, as along this line there is an abundance of stone 

 quarries, and the loch shore yields valuable natural sections. At Aith, W., at Sand wick 

 Manse, W. 10° N., at Clumly Loch, W., at Lyking, W. 10° N., finally at Bookan, 

 in one of the most prolific in fossils of all the quarries in Orkney, we have an 

 unbroken chain of west dips, which ends only in the isthmus on which are placed the 

 Standing Stones of Stenness. Along the shore of the Harray Loch, from Voy to Brodgar, 

 the section is very complete, and not quite so simple as the inland exposures would have 

 led us to expect. The rocks which form the Ness of Tenston have indeed a prevalent 

 west dip, but sometimes roll to the east, while reefs of vertical beds run out into the 

 loch in a direction N. 10° W., and everywhere there is much contortion and slickensiding, 

 the organic matter of the dark flags having been deposited as a brightly polished layer 

 on the bedding planes. These are the symptoms which everywhere in Orkney indicate 

 the presence of a considerable fault ; and as these broken rocks of Tenston Ness occupy 

 a belt of the breadth of about half a mile, the dislocation can hardly be supposed 

 to be a trivial one. Traced southwards, the same phenomena are to be seen in the 

 rocks around the Bridge of Waithe. From Garson farm, near Stromness, by Bu Point, 

 to the Bridge of Waithe, the rocks are folded into many sharp little anticlines and 

 synclines, with mostly a north and south strike. At the bridge and down the Ireland 

 shore by Cumaness, reefs of vertical slickensided and crushed rock are seen in several 

 places running N. 10° W., and from here along the shore to Houton we have again a 

 continual and rapidly changing succession of little folds (as was remarked by Messrs 



